$ A = \left[\begin{array}{rrr}0 & -1 & 2 \\ 1 & 2 & -1\end{array}\right]$ $ D = \left[\begin{array}{rr}-1 & 2 \\ -1 & -2 \\ 3 & 1\end{array}\right]$ What is $ A D$ ?
Explanation: Because $ A$ has dimensions $(2\times3)$ and $ D$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ A D = \left[\begin{array}{rrr}{0} & {-1} & {2} \\ {1} & {2} & {-1}\end{array}\right] \left[\begin{array}{rr}{-1} & \color{#DF0030}{2} \\ {-1} & \color{#DF0030}{-2} \\ {3} & \color{#DF0030}{1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ A$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ A$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ A$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rr}{0}\cdot{-1}+{-1}\cdot{-1}+{2}\cdot{3} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ A$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{-1}+{-1}\cdot{-1}+{2}\cdot{3} & ? \\ {1}\cdot{-1}+{2}\cdot{-1}+{-1}\cdot{3} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ A$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{-1}+{-1}\cdot{-1}+{2}\cdot{3} & {0}\cdot\color{#DF0030}{2}+{-1}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{1} \\ {1}\cdot{-1}+{2}\cdot{-1}+{-1}\cdot{3} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{0}\cdot{-1}+{-1}\cdot{-1}+{2}\cdot{3} & {0}\cdot\color{#DF0030}{2}+{-1}\cdot\color{#DF0030}{-2}+{2}\cdot\color{#DF0030}{1} \\ {1}\cdot{-1}+{2}\cdot{-1}+{-1}\cdot{3} & {1}\cdot\color{#DF0030}{2}+{2}\cdot\color{#DF0030}{-2}+{-1}\cdot\color{#DF0030}{1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}7 & 4 \\ -6 & -3\end{array}\right] $